1. Field of the Invention
The present invention relates to a bias circuit for a radio frequency power amplifier, and more particularly, to a bias circuit for improving linearity of a radio frequency power amplifier.
2. Description of the Related Art
FIG. 1 is a schematic diagram showing an example of a conventional bias circuit for a radio frequency (RF) power amplifier. Referring to FIG. 1, in a conventional resistive bias circuit, a bias voltage source Vbias is supplied to a base of an RF transistor 102 through a bias resistor 104, thereby providing a base current of the RF transistor 102. A capacitor 106 is connected between an RF input port of the RF power amplifier and the base of the RF transistor 102, thereby coupling an RF input signal (but not a direct-current signal) to the base of the RF transistor 102. Through an output matching circuit 108, a collector of the RF transistor 102 functions as an output port of the RF power amplifier. The resistive bias circuit has a disadvantage of providing a limit control over the bias current. For example, if the bias resistor 104 has a small resistance, temperature variations will cause unacceptable fluctuations in the quiescent current unless the bias voltage source Vbias also changes with temperature. On the other hand, if the bias resistor 104 has a large resistance, the RF transistor 102 will have insufficient bias current at high drive levels or have a large quiescent bias current which is undesirable.
FIG. 2 is a schematic diagram showing another example of a conventional bias circuit for an RF power amplifier. A conventional active bias circuit shown in FIG. 2 is an improvement of the resistive bias circuit shown in FIG. 1. Referring to FIG. 2, the active bias circuit includes a bias transistor 202 for allowing the RF transistor 102 to draw varying amounts of bias current depending on the RF drive level while still maintaining a low quiescent current level. The bias voltage source Vbias is supplied to a base of the bias transistor 202 through the bias resistor 104. The bias transistor 202 is an emitter-follower-type transistor. A collector of the bias transistor 202 is connected to a DC voltage Vcc. The active bias circuit further has an advantage of low impedance.
However, the active bias circuit shown in FIG. 2 has a disadvantage that the bias transistor 202 may be driven into a saturation state. More specifically, when the RF transistor 102 are driven to output a high power, part of the RF input signal is reflected from the collector of the RF transistor 102 back to the base of the RF transistor 102, and may further enter the active bias circuit. As a result, the bias transistor 202 is driven into the saturation state by the part of the RF input signal coupled back to the bias transistor 202, causing its operation to become more nonlinear. Under this circumstance, the active bias circuit cannot follow the RF input signal to provide the RF transistor 102 with a linear bias current.